1. Field of Invention
This invention relates to a fire resistant or non-flammable resin composition, and a coated electrical wire including an electrical conductor and a covering made of the composition.
2. Description of Related Art
Polyvinyl chloride, which has superior flame resistance, is conventionally used as a coating material for electrical wire that is used in vehicles or electrical/electronic equipment. To obtain required characteristics in the wire, such as mechanical properties including wear resistance and tensile strength, along with flexibility and processability, additives such as plasticizers or stabilizers are added to the polyvinyl chloride in varying amounts.
Polyvinyl chloride has low flammability. However, it contains halogen, so when a vehicle or electrical/electronic equipment employing wire coated with polyvinyl chloride is burned, a harmful halogen gas is released into the atmosphere, thus causing environmental pollution.
In recent years, research has been conducted on fire resistant resin compositions that do not contain halogen. For example, a halogen-free resin composition disclosed in JP-A-5-301996 contains a mixture of a polyolefin and a high-density polyethylene, with a metal hydroxide added to the mixture as a fire resistant agent.
Another halogen-free fire resistant resin composition is disclosed in JP-B-7-110912. This composition contains a mixture of a thermoplastic elastomer and a polyolefin having a low crystallisability, with an inorganic fire resistant agent added to the mixture.
The electrical wire disclosed in JP-B-7-78518 includes a conductor covered with a cross-linked resin composition containing a mixture of a polyolefin having a melting point of not less than 120xc2x0 C. and a carboxylic acid-modified polymer, with a surface-treated magnesium hydroxide added to the mixture.
However, in the case of the composition disclosed in JP-A-5-301996, it is necessary to add a large amount of metal hydroxide to the mixture of the polyolefin and the high-density polyethylene to achieve sufficient fire resistance, so that the composition is self-extinguishing. Due to the quantity of metal hydroxide, the composition has poor mechanical strength demonstrated by low wear resistance and tensile strength.
It is possible to increase the content of the high-density polyethylene, which is crystallizable and has comparatively high hardness, to improve the mechanical characteristics of such a composition. However, if the quantity of high-density polyethylene is increased, the composition has a small amount of non-crystalline material, and it is only possible to add small amounts of the fire resistant agent to the mixture. Consequently, the composition has reduced fire resistance and flexibility. Further, when the composition is used as a covering material for electrical wire, it has very poor processability and extrusion moldability. Thus, the composition is not satisfactory.
In the case of the composition disclosed in JP-B-7-110912, it is also necessary to add large amounts of an inorganic fire resistant agent to the mixture to achieve a self-extinguishing property. Consequently, as with the composition of JP-A-5-301996, the composition has poor mechanical strength as demonstrated by poor wear resistance, tensile strength, and the like. Further, due to the quantity of the fire resistant agent, the flexibility of the thermoplastic elastomer of the composition deteriorates. If the amount of the polyolefin is reduced to improve the mechanical characteristics of the composition, only a small amount of a non-crystalline material remains, and only a small amount of the fire resistant agent can be added. Consequently, the composition has poor fire resistance and does not possess the characteristics desired in wire manufacture.
In the case of the electrical wire of JP-B-7-78518, the resin composition of the covering is cross-linked. Thus, it is necessary to introduce a cross-linking step into the process of producing covered electrical wire. This additional step increases the number of manufacturing steps, in turn increasing the cost of manufacture.
Therefore, an object of this invention is to provide a fire resistant resin composition that does not generate halogen gas when burnt. Another object of this invention is to provide a composition having good mechanical characteristics including wear resistance, tensile strength, tensile elongation, and the like. An additional object of this invention is to provide a composition having good flexibility and processability.
Another object of this invention is to provide an electrical wire including an electrical conductor and a coating made of a fire resistant resin composition.
According to this invention there is provided a fire resistant resin composition including synthetic resin components:
(a) from about 60 to about 97 parts by weight of a propylene resin having propylene as its main monomer component by weight, and
(b) from about 3 to about 40 parts by weight of a styrene-based thermoplastic elastomeric resin modified with an unsaturated carboxylic acid or a derivative thereof,
components (a) and (b) being free of halogen, the total amount of components (a) and (b) being 100 parts by weight and there being no other synthetic resin components included in the composition,
the composition further including:
(c) from about 30 to about 200 parts by weight of a fire resistant metal hydroxide per 100 parts by weight of components (a) and (b).
The invention further provides a method of making the resin composition by combining and mixing components (a), (b) and (c). Component (b) should preferably be modified with the unsaturated carboxylic acid or its derivative, before it is mixed with components (a) and (c).
The composition of this invention does not contain halogen, so it does not generate halogen gas when burned. In addition, the composition includes from 60 to 97 parts by weight of propylene resin, an olefin having a comparatively high melting point, making it possible to improve the heat resistance of the resin composition without cross-linking the polymer.
The composition contains from 3 to 40 parts by weight of styrene thermoplastic elastomer modified with an unsaturated carboxylic acid or derivative, so the composition is flexible. Further, a strong bonding interface is formed between the polar metal hydroxide, which acts as a fire resistant agent in the composition, and the modified styrene thermoplastic elastomer. Therefore, it is possible to improve mechanical characteristics of the resin composition including wear resistance, tensile strength, and tensile elongation without reducing the flexibility of the composition.
The resin composition of this invention is highly processable and moldable because the composition possesses a good balance of mechanical characteristics and flexibility. In addition, because the resin composition contains from 30 to 200 parts by weight of the metal hydroxide, it provides satisfactory fire resistance.
In various preferred embodiments, the styrene thermoplastic elastomer modified with the unsaturated carboxylic acid or the derivative thereof is a maleic anhydride-modified styrene thermoplastic elastomer. The metal hydroxide is preferably magnesium hydroxide. In embodiments where these components are employed, the resin composition has excellent mechanical characteristics such as wear resistance.
The electrical wire of this invention includes a conductor and the fire resistant resin composition applied to a peripheral surface of the conductor. It is preferable that the covering made of the resin composition has a thickness of from about 0.2 mm to about 0.3 mm.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of this invention.